Need help for a brushless conversion

[kck741]

I thought if you limit the amount of current to a brushless motor,it hold back current in the ESC therefor leading to heat,and then fail.thats why brussless motors perform best at there peak!

[ED66677]

I'm not sure it is really possible to limit current easily, in DC current system limitating current means reducing voltage or increasing resistance, and I'm not talking of current limitation but reducing amp draw, once again, a 8XL is given for 88amp max (under 40volt max as far as I remember, more than 3000Watt!!!), that doesn't means you can't run it at 30 or 40 amp, such amp draw represent a quite nice power under 6 cell, think about it, a 600Race motor is given for 110 watt max power, with any brushless motor that can drain that much amp under 6cell you'll have twice this power, the big difference with the 600 class is the efficiency, around 70% for brushed motor, close to or more then 90% for brushless, and you are right on that point, because your 600 at 30amp under 6 cell is probably around 50%! choice is made!

I made a simulation with FECalc, 600 7,2V, 6 cell, K43,5 carbon prop (surface piercing situation, so only 22mm more or less sub prop theorycaly), 32amp, 17,6mph, 10500rpm and 55% efficiency!!! you'r already out of range with the 600!
Same for the 8XL with a K40,5, FECalc says 32amp,21,6mph, 13799rpm and 85,8% efficiency, a 9L for example will have a higher kv that, in surface piercing scheme, gives better speed and equivalent amp draw and efficiency but at 18000rpm!!!

Remember that double power doesn't means double speed!!! it's some kind of exponential!!!

My advice is to take a powerfull motor that you'll be able to use on a better hull, and use it in you given condition, 30-40amp 6-8cell, at 40 amp the 8XL will not require water cooling!!!

[kff]

Hello ED,
thanks
There is a motor calculator for boat ???? FEcalc ????? I don't know it : where can I find it please ???

For my electric aircraft I use Motocalc for years and it a great tool.

[kff]

I've just download FECalc, will see how it works.

[ED66677]

you can take any data from motor, prop, cell and add them to the txt file, but do so only one after the other, if you make any mistake FeCalc won't start!!!

Attention FeCalc is destined for surface piercing prop and not sub, so for aproximation divide prop diameter per 2

[kff]

devide by 2 ????? for sub prop !!!!

No, x2 I think because sub prop need more power than surface prop !!!!!

I've done a simulation and for a speed 600 7.2 (I have created a new line in the database with my motor specs) , I've found with a octura x438 the same current than reality (27 amp with 7 cells).

the x438 is a 38 mm diam and my prop is a 30 mm prop !!!!!!!!!!!

[ED66677]

nope! divided a surface piercing prop only has half of it in water, so one blade must be equivalent to the 2 blades of a sub prop!!!

If you make a simulation with a K45 in FeCalc then you have to consider that the appropriate sub prop must be around 25mm!!!

[kff]

Yes you're right
but in my case to find the reality measurement of 27Amp at full speed I have to take a X438 octura witch is a 38 mm prop and in my case I've got a 30 mm prop and not a 17 mm : there is a big difference.

Never mind. in my case (je fais un calcul relatif et non absolu : le but est de trouver le moteur qui me fera rester dans un ampérage résonnable : sorry english readers it 's sometimes difficult for me to speak technical english)

If a take a FG 540 12L instead of my brushed motor , with the same parameter , I am in the 27 amp range but with 30W more power to the prop and 2 mph more.

If a take a FG 540 10L : 38 amp and 90w more power to the prop and 5 mph more.

(I won't take a XL because it is too heavy et je ne veux pas plomber le bateau)

that's what i'm looking for !!!!

what do you think about that.

have a nice weekend
see you on monday

[ED66677]

somthing you have to take to account is the fact that if you don't have a tool like the EagleTreeMicroPower E-Logger or DPR, you can't really be sure of what's going on in the boat, you originaly say that your setup is pulling 27amp, how did you do that? with a multimeter and static boat in a pond? that's is certainly a bit far away from reality, if my guesses are correct, you then did measure while the boat was not moving on water and then create more resistance to the prop, when running your setup probably pull only 20amp!!!

the XL is only 100g heavier than the L, won't be a big issue!!!

[kff]

Hello ED,
thanks for our help,

I'm sure about my setup because I made few runs with a wattmeter inboard.
I fly electric aircraft and heli for many year and I have every tools (wattmeter, DPR II ... ) to check a setup.

with 7 cells
27,15 amp max : 8.15V min at full speed with slow accéleration
29,54 amp max : 7,95V min fast acceleration

with 6 cells
20,42 amp max : 6.66V min at full speed with slow accéleration
23,47 amp max : 6.50V min fast acceleration

best regards

[ED66677]

OOPS!!!
cool then!

[kff]

hello ED,
You could'nt know !!!!!
Many thanks for your help, I've got now a idea on how it's work.
time to find the right motor, and I'm not in a hurry.
I'll tell you my choice and some DPR infos if the first runs.

regards

[kff]

Hello,

just happy to tell you that my mono hull brushless conversion works great.

I bought a feigao 540 12L (2050 kv) at : offshoreelectrics.com
(thanks to Steven for quick postage)

I put a aircraft controller (Xpower 60 bec : same as hextronik) : converted with home-made cooling jacket.

I use my stock propeller (immerged) and my stock 7-cells RC2000 nicd.

The boat is more powerfull with better speed for 5 minutes.

motor and controller run cold :

I use my DPR : power is 220W and amp is 27A , so it is about the same than my old speed 600 but with a better efficiency.

I am happy about this conversion for few dollars

REGARDS
Olivier